1. Field of the Invention
The present invention relates to a method of measuring the coefficient of friction established between a slider or a contacting member and a receiving surface in contact with the slider or the contacting member, and in particular, to a method of measuring the coefficient of friction between a head slider carrying an electromagnetic transducer and the surface of a magnetic disk in a magnetic medium drive or storage device.
2. Description of the Prior Art
A contact head slider is proposed in the technical field of a magnetic medium drive or storage device such as a hard disk drive. The contact head slider designed to keep contacting the surface of a magnetic recording medium is supposed to contribute to realization of a still higher recording density in the magnetic medium drive. It is required to evaluate the friction between the head slider and the surface of the magnetic recording medium at a higher accuracy when the realization of the contact head slider is intended.
For example, Japanese Patent Laid-open No. 2-198337 discloses a conventional method of measuring the coefficient of friction. The method comprises urging a slider against the surface of a magnetic disk under a constant urging force when the magnetic disk rotates at a constant velocity. The friction is then measured between the urged slider and the surface of the rotating magnetic disk. The coefficient of friction is simply derived from calculation based on the detected friction and the magnitude of the urging force.
A lubricating agent or oil usually spreads over the surface of the magnetic disk so as to suppress the abrasion of the head slider and/or the magnetic disk in the magnetic disk drive. The lubricating agent or oil is supposed to establish an adsorption acting on the lightweight slider. The slider is supposed to always suffer from the adsorption irrespective of application of the urging force when the slider contacts the surface of the magnetic disk. The conventional method of measuring inevitably results in derivation of the coefficient of friction larger than the true or actual value because of the effect of the adsorption. This is because the detected friction acting on the slider includes a component induced under the influence of the adsorption. The conventional method of measuring the coefficient of friction fails to take account of the effect of the adsorption.
It is accordingly an object of the present invention to provide a method of measuring the still accurate coefficient of kinetic friction for a slider or contacting member receiving adsorption from a receiving surface.
According to a first aspect of the present invention, there is provided a method of measuring a coefficient of friction, comprising: moving a receiving surface; applying to a slider a vertical driving force perpendicular to the receiving surface until the slider contacts the receiving surface; measuring an adsorptive tangential force transmitted from the receiving surface to the slider at a moment when the slider contacts the receiving surface; and measuring an overall tangential force transmitted from the receiving surface to the slider when the slider is urged against the receiving surface with an urging force.
The adsorptive tangential force is supposed to represent a separate tangential force generated solely by the adsorption without a component under the influence of the friction generated by the urging force. The measurement of the adsorptive tangential force simply leads to observation or analysis of the independent adsorption. When the measured adsorptive tangential force is taken out of the overall tangential force, it is possible to derive the friction generated solely by the urging force or normal reaction between the receiving surface and the slider without the influence of the adsorption. The thus derived pure friction contributes to derivation or calculation of the still accurate coefficient of friction between the receiving surface and the slider. The influence of the adsorption can totally be eliminated in derivation or calculation of the coefficient of friction. Moreover, the magnitude of the adsorption can additionally be derived.
In estimating the coefficient of friction, the method may include: calculating a differential between the overall and adsorptive tangential forces; and calculating a differential between the vertical driving force keeping the contact of the slider against the receiving surface and an initial contact vertical driving force, for example. The differential between the vertical driving forces is supposed to represent the urging force urging the slider against the receiving surface. Accordingly, if the differential between the tangential forces is divided by the differential between the vertical driving forces, the coefficient of dynamic friction can be derived. Here, the initial contact vertical driving force represents a vertical driving force at the moment when the slider has just contacted or touched the receiving surface. The differential between the overall and adsorptive tangential forces is supposed to correspond to the pure friction without the influence of the adsorption. As a result, the still accurate coefficient of dynamic friction can be derived.
It is preferable to periodically vary the vertical driving force in the aforementioned method of measuring. The method is allowed to repeat step of moving the slider toward the receiving surface until the slider contacts the receiving surface and the step of urging the slider against the receiving surface. Measurement of the adsorptive and overall tangential forces is also repeated. It is possible to eliminate the effect of any disturbance to the utmost so as to derive a still further accurate coefficient of dynamic friction.
The adsorptive and overall tangential forces may be calculated based on a tangential spring constant of a support spring supporting the slider and a displacement of the slider in a direction along the receiving surface, for example, since the product of the tangential spring constant and the displacement of the slider is equivalent to the tangential force. A laser Doppler velocimeter (LDV) can be employed to detect the displacement of the slider, for example.
The method of measuring may be designed to continuously increase the vertical driving force so as to generate the urging force. The variation is measured in the vertical driving force. Also, the variation is measured in the tangential forces from the moment when the adsorptive tangential force has occurred until the overall tangential force reaches a maximum. The comparison between the variations serves to reveal the initial contact vertical driving force established at the moment when the adsorptive tangential force starts to solely act on the slider.
The initial contact vertical driving force can be calculated based on a vertical spring constant of a support spring supporting the slider and a displacement of the slider in a direction toward the receiving surface, for example, since the product of the vertical spring constant and the displacement of the slider is equivalent to the vertical driving force. In this case, an electrostatic actuator can be employed to generate the displacement of the slider, for example. If a predetermined relationship can be determined between the applied voltage and the driving force in the electrostatic actuator, the driving force can be estimated based on the magnitude of the applied voltage. Alternatively, the initial contact vertical driving force may be calculated based on the vertical spring constant of the support spring and the displacement of the slider in the direction vertical to the receiving surface. A laser Doppler velocimeter may be employed to detect the displacement of the slider in the direction vertical to the receiving surface.
According to a second aspect of the present invention, there is provided a method of measuring a coefficient of friction, comprising: moving a receiving surface; urging a slider against the receiving surface with a first load; urging the slider against the receiving surface with a second load larger than the first load; and measuring a tangential force transmitted from the receiving surface to the slider when the first and second loads are respectively applied to the slider.
The adsorption of the receiving surface influences equally to the tangential force induced by the first load and the tangential force induced by the second load. The differential between the tangential forces is supposed to reflect the magnitude of the tangential force induced solely by the friction in response to the normal reaction from the receiving surface. The influence of the adsorption can thus be eliminated. If the coefficient of dynamic friction is derived from the thus estimated friction between the receiving surface and the slider, it is possible to derive or calculate the still accurate coefficient of dynamic friction, independent of the effect of the adsorption between the receiving surface and the slider. In calculating the coefficient of dynamic friction, a predetermined differential is preferably set between the first and second loads, for example. The divide of the differential between the tangential forces by the predetermined differential between the loads leads to derivation of the coefficient of dynamic friction.
In this case, it is preferable to periodically vary the load within a range including the aforementioned first and second loads. The first and second loads may alternately be applied to the slider. Measurement of the tangential force is repeated. It is possible to eliminate the effect of any disturbance to the utmost so as to derive a still further accurate coefficient of dynamic friction.
The tangential force may be calculated based on a tangential spring constant of a support spring supporting the slider and a displacement of the slider in a direction along the receiving surface, for example, since the product of the tangential spring constant and the displacement of the slider is equivalent to the tangential force. A laser Doppler velocimeter (LDV) can be employed to detect the displacement of the slider, for example.
In realizing the aforementioned methods, a specific head slider may be incorporated within a magnetic disk drive. The head slider may include: a slider body having a bottom surface opposed to a magnetic disk; a contacting member supported on the slider body for movement relative to the slider body; and a driving power source disposed on the slider body so as to generate an urging force for urging the contacting member against the magnetic disk.
Employment of the head slider enables measurement for the coefficient of dynamic friction in a condition similar to the actual environment of a general head slider to be incorporated in a magnetic disk drive. For example, employment of an electrostatic actuator as a driving power source enables generation of a tiny or smaller vertical driving force or a load in a facilitated manner. The electrostatic actuator serves to achieve an accurate observation or evaluation for the behavior of the head slider.
A capacitance displacement sensor may be employed to detect the displacement of the contacting member incorporated in the head slider. The capacitance displacement sensor may be mounted on the slider body. When the contacting member is allowed to receive electromagnetic transducers or read/write head elements, it is possible to measure the coefficient of dynamic friction and the adsorption in a condition still similar to the actual environment of a general magnetic disk drive.
The aforementioned methods can be employed to measure the coefficient of friction for any types of the receiving surface including a disk or recording medium in addition to the magnetic recording disk.